The present invention relates to a method for the total precipitation of valuable metals from a leaching solution acified by sulphuric acid and which solution is essentially free from impurities and contaminants, by neutralising the solution with lime and/or limestone. The invention can be applied in particular with leaching solutions obtained in bioleaching processes, although it is not restricted to such leaching solutions.
In the total precipitation of valuable metals and contaminant metals from an acid leaching solution obtained when processing hydrometallurgically ores and other raw materials and intermediate products that contain valuable metals, by neutralisation with lime or limestone, it is necessary in known processes to use a given surplus of the neutralisation agent in order to obtain a satisfactory precipitation kinetic and thus achieve a sufficiently rapid precipitation rate. A lime surplus has been found to result in highly effective separation of metal and arsenic impurities for instance, as described in EP-B-0622338.
In respect of bioleaching processes for winning zinc from an iron-free solution, it is suggested in an article by Sandstrom et al (IBS ""97 Proceed. Biomine Int. Conf., Sydney 1997) that an upconcentration of the zinc content by total precipitation is effected by neutralisation, whereafter the zinc content is won from the precipitate by an electrowinning process via leaching with return acid from the electrolysis circuit. However, it is not possible to use a lime surplus in such a process, since this would require an external addition of sulphuric acid to the leaching circuit, which is undesirable in view of the fact that the circuit is a closed circuit where the acid requirement for dissolving the zinc content shall be balanced by the acid produced in the metal precipitation process in the electrolysis stage.